1. Field of the Invention
The present invention relates generally to a method and device for removing a liquid disposed in a material.
2. Description of the Related Art
Mechanical pressing is commonly used for drying various water-containing materials, for example bark and sludge.
The drum press comprises a rotating drum containing an internal parallel eccentric press roll. When the drum rotates, the material to be processed is pressed between the press roll and the drum. Usually the drum is perforated, and the liquid leaves the pressed material through the drum.
Using the drum press, quite high dry substance concentrations can be reached. For bark, depending on the type of wood, the concentration is preferably 40-45%, and optimally 45-50%. For sludge, the concentration is typically somewhat lower, depending on the type of sludge and above all on the biological sludge content.
If still higher dry substance concentrations are desired, the temperatures of the material to be pressed can be increased. As known, the material can be heated, for example, by feeding steam into the material before or during pressing. The latter is done typically in a screw press, where feeding steam into the material through holes in the screw shaft is easy. Examples of the higher final dry substance concentration achieved by heating can be found, for instance, in patent application FI 934893 and SU inventor's certificate 1005836, where pulp suspension is heated before filtering. Heating of the material increases the final dry substance concentration, for example, because warm water has lower viscosity and so exhibits a lower flow resistance when it leaves the material. However, it is not often that pulp suspensions shall be heated, as they typically are not already very hot after the digesting process.
However, the heating processes of the prior art are not optimal since the gain collected by them has remained quite small compared with the required energy consumption, so heating the water contained in the material to be dried demands large amounts of energy. Thus one should observe, with respect to the above-mentioned reference publications, that the entire flow of the material that is to be dried is heated.